the syntax extends is just syntax sugar (httpfs://en.wikipedia.org/wiki/Syntactic_sugar) and is treated equivalently to mixes. You can think of it as if everytime you type extends A mixes B, C the platform treats it as mixes A, B, C.

The real rule is that you can only MIXIN 1 persistable type.

When you mixin a persistable type, the documentation is rendered as extends

This might change in the future where extends and mixes are really different.